Implement slashing protection interchange format (#1544)

## Issue Addressed

Implements support for importing and exporting the slashing protection DB interchange format described here:

https://hackmd.io/@sproul/Bk0Y0qdGD

Also closes #1584 

## Proposed Changes

* [x] Support for serializing and deserializing the format
* [x] Support for importing and exporting Lighthouse's database
* [x] CLI commands to invoke import and export
* [x] Export to minimal format (required when a minimal format has been previously imported)
* [x] Tests for export to minimal (utilising mixed importing and attestation signing?)
* [x] Tests for import/export of complete format, and import of minimal format
* [x] ~~Prevent attestations with sources less than our max source (Danny's suggestion). Required for the fake attestation that we put in for the minimal format to block attestations from source 0.~~
* [x] Add the concept of a "low watermark" for compatibility with the minimal format

Bonus!

* [x] A fix to a potentially nasty bug involving validators getting re-registered each time the validator client ran! Thankfully, the ordering of keys meant that the validator IDs used for attestations and blocks remained stable -- otherwise we could have had some slashings on our hands! 😱
* [x] Tests to confirm that this bug is indeed vanquished

Cargo.lock

@@ -23,6 +23,7 @@ dependencies = [
  "libc",
  "rand 0.7.3",
  "rayon",
+ "slashing_protection",
  "slog",
  "slog-async",
  "slog-term",
@@ -3057,6 +3058,7 @@ dependencies = [
  "futures 0.3.5",
  "lighthouse_version",
  "logging",
+ "slashing_protection",
  "slog",
  "slog-async",
  "slog-term",
@@ -4935,6 +4937,10 @@ dependencies = [
  "r2d2_sqlite",
  "rayon",
  "rusqlite",
+ "serde",
+ "serde_derive",
+ "serde_json",
+ "serde_utils",
  "tempfile",
  "tree_hash",
  "types",